(1) Field of the Invention
The present disclosure is related to an ultrasound signal processing device, and an ultrasound diagnostic device equipped with the ultrasound signal processing device. In particular, the present disclosure relates to receive beam forming in an ultrasound signal processing device.
(2) Description of the Related Art
Typically, an ultrasound diagnostic device transmits ultrasound towards the inside of a subject via an ultrasound probe (referred to in the following as a “probe”), and receives reflected ultrasound (an echo) via the probe. The reflected ultrasound is generated within the subject due to tissues in the subject having different acoustic impedances. Further, an ultrasound diagnostic device generates an ultrasound tomographic image based on electric signals acquired through the reception of the reflected ultrasound, and displays the ultrasound tomographic image on a monitor (referred to in the following as a “display unit”). An ultrasound tomographic image shows the structures of tissues inside the subject. Ultrasound diagnostic devices are widely used for the imaging diagnosis of subjects, for having low invasiveness and achieving real-time observation of tissues through tomographic images and the like.
A typical method applied in conventional ultrasound diagnostic devices for forming signals based on received reflected ultrasound (i.e., receive beam forming) is delay-and-sum beam forming. One example of delay-and-sum beam forming can be found disclosed in pages 42-45 of “Ultrasound Diagnostic Device”, written by Masayasu Itou and Tsuyoshi Mochizuki and published by Corona Publishing Co., Ltd (Aug. 26, 2002). According to this method, transmission beam forming (i.e., transmission of ultrasound by a plurality of transducer elements towards the inside of the subject) is typically performed such that a transmitted ultrasound beam converges (focuses) at a predetermined focal depth inside the subject. Further, according to this method, measurement points are always set along the central axis of the transmitted ultrasound beam, as illustrated in FIG. 27. Due to this, one ultrasound transmission event generates only one or a few acoustic line signals along the central axis of the transmitted ultrasound beam, and thus, reflected ultrasound is not utilized in an efficient manner. In addition, with this method, it is also problematic that an acoustic line signal acquired from a measurement point distant from the transmission focal point has low resolution and low S/N ratio.
Meanwhile, a receive beam forming method is being proposed that utilizes a so-called synthetic aperture method to yield high quality images not only from near the transmission focal point but also from areas other than near the transmission focal point. One example of receive beam forming utilizing the synthetic aperture method can be found disclosed in pages 395 through 405 of “Virtual Ultrasound Sources in High Resolution Ultrasound Imaging”, S. I. Nikolov and J. A. Jensen, in Proc, SPIE—Progress in Biomedical Optics and Imaging, Vol. 3, 2002. According to this method, delaying is performed taking into consideration both a propagation path of ultrasound and the time amount required for reflected ultrasound to arrive at a transducer element by travelling along the propagation path. Thus, the method achieves receive beam forming making use of not only reflected ultrasound from an area of an ultrasound main irradiation area near the transmission focal point but also reflected ultrasound from areas of the ultrasound main irradiation area other than the area near the transmission focal point. Due to this, the method enables generating, from one ultrasound transmission event, acoustic line signals covering the entire ultrasound main irradiation area, including areas far from the transmission focal point. In addition, the synthetic aperture method enables setting a virtual transmission focal point based on multiple receive signals acquired for each measurement point through multiple transmission sessions. Thus, the synthetic aperture method enables acquiring an ultrasound image with higher resolution and higher S/N ratio than the receive beam forming method disclosed in “Ultrasound Diagnostic Device”.